1. Field of the Invention
The invention relates generally to electronic components and more particularly to variable resistive elements operable at microwave frequencies.
2. Discussion of Prior Art
Potentiometers, adjustable T-pad attenuators and numerous other applications of manually adjusted variable resistance elements are well known in audio frequency electrical arts and in radio frequency (RF) electrical arts. However, similar devices are unknown in microwave frequency arts. Yet, there exists a desire for manually adjusted resistive circuits in all of the electrical arts.
Variable resistance elements are typically bulky devices having large geometries entirely unsuitable for microwave applications. The problem of bulk and large geometry are essentially doubled when gang-adjustable variable resistance elements are required, such as in a T-pad attenuator circuit. One example of a gang-adjusted variable resistance element suitable for audio frequency application is the ALLEN-BRADLEY type BT Variable Attenuator. However, the type BT device is 0.5 inches in diameter and comprises two circular resistive elements coaxially arranged on a single shaft for adjustment. The large diameter of this unit results in a resistive element quite long with respect to microwave signal wavelengths. Therefore, the behavior of this unit at microwave frequencies would include transmission line effects, rendering it useless at those frequencies. For example, the characteristic impedance of the unit would be unacceptably high and would vary with frequency.
Mechanical constraints on a shaft-adjusted variable resistance element are such that a device usable in the microwave frequency range is impractical. For example, consider a single-turn design, based on a resistive element disposed along the circumference of a circle having a diameter of about 0.1 inches. In such a device, the resistive element is approximately 0.3 inches long. The length of the resistive element may cause a problem due to the resulting characteristic inductance of the unit. If two such devices are arranged back-to-back and gang-adjustable, such as in the audio unit described above, then inter-device interconnect lengths may be minimized. However, such a unit would have to be mounted in a vertical position which is highly likely to cause radiation or reception of noise. When the device is arranged so as to minimize its behavior as an antenna, for example by placing the device horizontally, interconnect lengths become long, causing inductance measured at microwave frequencies to be quite high. Also, even when arranged horizontally, one unit must be located directly above the other unit, and the upper unit behaves as an antenna.
The long interconnecting leads and the designs which do not take transmission line effects into account of the prior art result in extremely high standing wave ratios. Thus, the impedance of such devices vary with frequency to such a degree that the attenuation characteristics are useless at microwave frequencies.
Therefore, an adjustable miniature resistive element usable at microwave frequencies is desired. Furthermore, a miniature, dual, gang-adjustable resistive element usable at microwave frequencies is desired.